It has already been known well that the aromatic PAI copolymers being superior in heat resistance are generally obtainable by polycondensing an aromatic tricarboxylic acid anhydride or a derivative thereof and an aromatic diamine or a derivative thereof (e.g., Japanese Laid-Open Pat. Nos. 15637/1967, 19274/1969, 2397/1970, 4077/1974, 33120/1975, etc.).
Further a number of proposals have already been advanced as to the modifications of basic PAI copolymers by using an aromatic tricarboxylic acid anhydride or a derivative thereof and an aromatic dicarboxylic acid dichloride as acid components (e.g., Japanese Laid-Open Pat. Nos. 8894/1975, 121397/1975, 16908/1971, 12594/1974, 13240/1974, 26316/1074, 510882/1981, etc.). And it is also well known that special aromatic PAI copolymers having components A and B (Ar--simplex material) according to the present invention are synthesized by the reaction between a trimellitic acid monochloride anhydride and an amide-bond incorporating aromatic diamine represented by general formula: ##STR4## wherein Ar is ##STR5## and n is an integer from 1 to 500 (e.g., U.S. Pat. No. 3,984,375 and Japanese Laid-Open Pat. No. 10029/1973).
Japanese Laid-Open Pat. No. 13240/1974 discloses PAI copolymers wherein --Ar-- is a simplex material synthesized by a process consisting of the first step of the production of ##STR6## (these compounds are generally called aramidediamine hereinafter) by the reaction of an excess mole diamine and ##STR7## and the second step of the reaction between the above-obtained aramidediamine, ##STR8## and H.sub.2 N--Ar--NH.sub.2 and the third step of the ring closure reaction of the resultant product. Also Japanese Laid-Open Pat. No. 8894/1975 discloses a polyamide amic acid (ring-open precursor of PAI) synthesized by using two or three --Ar-- units in the same manner for developing a functional film.
But the heat stability, melt fluidity and other physical properties of the aromatic PAI copolymers generally proposed hitherto are not well balanced when they are melt-molded.
For example the PAI copolymers represented by the general formula: ##STR9## that are synthesized from trimellitic acid chloride anhydride and 3',3"-diaminoisophthalanilide are far from practicable as an engineering plastic material because the resulting moldings are inferior in stiffness and mechanical strength despite their heat resistance being enough for practical use. The PAI copolymers synthesized by using: ##STR10## instead of aramidediamine: ##STR11## as diamine materials are not substantially moldable by any screw-in-line type injection molding machine because of inferior heat stability on melting. And the PAI copolymers synthesized from: ##STR12## and a trimellitic acid chloride anhydride can only produce moldings inferior in mechanical strength, although they barely permit melt-molding. Moreover the polymer disclosed in Japanese Laid-Open Pat. No. 8894/1975 is a ring-open precursor of PAI copolymers, and cannot substantially be used for melt-molding because condensation water is generated in great quantities on heat-melting.
U.S. Pat. No. 3,984,375 also describes that copolymers containing two or more of radicals given the above may be particularly valuable in some instance. But there are so many combinations containing two or more radical and it doesn't make clear what kinds of combinations have valuable properties concretely.